It seems like people are too much into the "prevention" maintenance than the "required" maintenance. To me a required maintenance is what the manual says...an yes some of you call it a bS but i call it as is...seen plenty of 3 and 5 series with original t-case fluids, trans fluids and diff fluids as high as 250k miles...all this talk but I yet have to see some kind of result or investigation of these life time fluids after they have been drained? Anyone has done this? Its easy to say ohh yeah its bad becsaue its BLACK..really? Oil turning black has nothing to do with it being good or bad, but these are signs that the motor oil is doing its job of keeping dirt, and other combustion contaminants in suspension...
IMO e46 powertrain is pretty solid and thats all it matters other small stuff why fix it if its not broke...

There have been oil analysis done for the drive line fluids. Google is your friend on that one. Additionally, BMW did change their mind and say that you should change your transmission fluid at ~100k. Additionally, yes the point of oil is to keep contaminants in suspension but there is a saturation point where the oil is now more abrasive due to the contaminants than lubricating. That is why you change it.

As for why fix if it's not broke...because it costs much more than the $30-$50 in fluid to fix a broken diff/TC/transmission.

I've said it before and I'll say it again, these cars are cheap to maintain and very expensive to fix.